Mental health is not measurable and in rare cases mental illness remain unnoticed. So it is easy to help someone who is suffering from physical illness like nausea or fever but its very hard to help someone suffering from mental illness if you don't have certain knowledge. So it is important for you to know what you can do to help someone suffering from any kind of mental illness.
In cases where mental illness becomes noticeable and you see difference in behavior of your relative or friend then this is the time to help him. There are different methods to help them especially by your words and actions. The very first way to help them is to provide them with what they need if you can like if they need a glass of water or they need you to turn on the fan or anything else that involves action from your side.
The most difficult part is to calm them by your words because to calm them you have to understand what they want to say and then carefully selecting your words. These words must not confirm their opinion if it is not good for them and should not deny them even if they are wrong. As a good friend you should agree with them on different opinions that doesn't affect anyone's life like who is the best sportsman or who was the best teacher at school time.
The major problem is that sometime people do not realize that mental patient is actually ill they think that he or she can control his/her mind like us but that's not the case you have to understand that sometime its very difficult for the mental patient to get control of him when certain type of reaction to any stimuli starts. A logical person can react in a mad way to certain type of stimuli so it is always best for you to understand his/her situation.